Sculpt mode, Symmetry, and the 'H' hotkey
Dyntopo has a symmetrize button which will symmetrize your geometry on any axis.



If you pick the wrong direction you can undo and choose another.

While you don't need to do any sculpting in dyntopo to use this, you do need to enable it, which means you can loose UV's and vertex colours that can normally be lost when enabling dyntopo.